Fluorescent lighting has gained prominence over the last 20 years as a light source for motion picture production and other color critical imaging applications. The many advantages of low power consumption, low heat, lightweight fixture designs, quiet ballasts and high color rendering lamps have all contributed to an industry wide acceptance of the technology.
The more recent introduction of stable dimming technology has presented an unforeseen problem for lighting professionals in the imaging industries. As fluorescent lamps are dimmed the lamps shift in color temperature. The shift in color temperature is very different from dimming an incandescent. The difference is best viewed or understood when comparing the color tracking points of the two sources in a CIE color space. The CIE (1931) color space has a black body color temperature curve or a Planckian locus. The curve defines the color temperature of a black body emitter such as a lamp filament as it glows from darkness to its final brightness or operating voltage. In photographic terms, film would see a lamp going from a very orange light to a white light at its brightest dimmer setting. A fluorescent lamp on the other hand does not follow the Planckian curve. As a fluorescent is dimmed it wanders off the curve and falls below it. This is an area of the CIE color space that defines the amount of magenta in the spectrum. The only shift in spectrum when dimming a fluorescent is in the green/magenta range. Since correlated color temperature is a mathematical calculation the color temperature is represented as dropping in temperature when in fact, unlike an incandescent, it is only shifting along a vertical axis below the Planckian curve.
The color temperature shift of an incandescent is greater that a fluorescent. For example, in photographic terms a four f′ stop dimming range in incandescent will result in color temperature going from 3200K to 2164K; a drop of 1036 Kelvin. There will be no shift in the green/magenta spectrum. In a fluorescent the same dimming range will result in a shift from 3200K to 2735K a drop of only 465 Kelvin, however there is a marked decrease in green spectrum.
This type of spectral shift in the green results in digital camera or film technology rendering colors incorrectly. This can be most noticeable on skin tones. For example a more magenta light source makes a Caucasian skin tone appear not just warmer as it would with a dimmed incandescent but unnaturally magenta. If the skin tone were to be corrected electronically in postproduction the background image lit by an undimmed fluorescent would appear green. This condition is unacceptable.
In order to understand the color shift, it is important to understand the mechanics of how a fluorescent lamp is illuminated. A fluorescent lamp is made up of a blend of various phosphors applied to the interior wall of a tubular light source. The phosphor lights up when exposed to ultraviolet light. This ultraviolet light is achieved by establishing a plasma arc stream through a mercury vapor atmosphere in a tubular lamp. The plasma arc is an electron stream established between two cathodes at opposite ends of the lamp. If just the arc stream could be viewed, it would appear as a blue green light. On a spectral distribution chart the arc would appear to have a very high energy spike at around the 550 nanometer range.
The color rendering of a fluorescent lamp is defined and tailored to be correct at its maximum light output. This is also the point at which the lamp is experiencing the highest mercury vapor pressure. This is when the arc is at its most blue/green and the lamp is at its brightest.
As in an incandescent lamp, as a fluorescent lamp is dimmed, light output and Kelvin temperature drops. Unlike incandescent, as the fluorescent lamp cools the mercury vapor pressure within the lamp drops resulting in a lowering of the green spectrum and the overall color temperature. This drop in green makes a lamp appear more magenta. Photographers would use a photographic color meter such as a hand held Minolta® color meter or a Sekonic® color meter to measure the drop in color temperature. The meters would calculate the amount of additive green filtration necessary to bring the light back in line to what the spectrum was prior to dimming.
Fluorescent lamps have a long history of requiring color correction gels to absorb parts of the spectrum that render colors on film inaccurately. The down side of color correction gels or filters applied directly to a fixture is that the light takes on the coloration of the gel/filter. That is to say, human eyes perceive the colored gel more so than the imaging technology that now renders or sees the light correctly. This hinders artists such as art directors or cinematographers from accurately evaluating and appreciating how the range of colors and tones will reproduce on film or digitally.
It is known in the art (e.g., U.S. Pat. No. 7,014,336) to provide a collection of LEDS representing the range of visible light to be individually attenuated in such a way as to simulate existing alternate light sources and their distinct spectral curves. This patent also shows an embodiment of a tubular light source populated with multiple LEDS to simulate and be used in place of a fluorescent tube. The patent also reveals a system of monitoring a given source spectrally and then extrapolating a matching spectrum using an array of LEDs representing the visible light range. However, this patent does not appear to contain any teachings with respect to improving color performance of a dimming fluorescent lamp such that its color spectrum and color temperature are maintained as the lamp is dimmed, or for otherwise correcting the light output from a fluorescent lamp.
Academy Award winning Kino Flo Lighting Systems in Burbank Calif. developed full spectrum fluorescent lamps that render colors accurately for various imaging applications. These lamps eliminated much of the color corrective filtering that was required for architectural lamps with deficient spectrums. The industry has noted that as fluorescent lamps dim they shift in color temperature and light output drops. Because each fixture can be dimmed to a different level, the degree of color shift can vary greatly from fixture to fixture. For a lighting director to add color correction gel or filters to all the dimmed fixtures would require a great deal of time and expense to determine the degree of filtration necessary. The discoloration of the light as a result of gelling further alienated artists from wanting to dim fluorescent lamps. As a result dimming fluorescent fixtures have a limited acceptance rate amongst most film or TV lighting professionals.